I've seen videos on how to water-candle an egg. Don't know how accurate it is, but you might try it before giving up. Basically, you first check the egg carefully for ANY external pips or cracks - if there are, then DO NOT proceed. If the egg is still fully intact, you can place it in a cup of warm water. It will float. Then watch the surface of the water. If the chick is alive, the water jiggles or wiggles a bit. If it stays perfectly smooth, dead chick.
(Do NOT attempt to help the hatch if the chick hasn't pipped externally. Trust me. It does not work out well at all. Imagine a time bomb filled with rotten egg.....)
OTOH, hens are very good at detecting a bad egg, and will reject it. If she's still sitting, it's a good sign.